The difference is that Stein, as a member of the Federal Open Markets Committee, has a vote on whether the Fed should try and pop that bubble, which it has had an important hand in creating by building up an unprecedented $3 trillion bond portfolio.

Investors are expecting the Fed to keep adding to that portfolio -- potentially inflating the bubble -- for another six months to a year. Any hint that the Fed may hit the brakes sooner than that could cause a sharp rise in bond yields, in anticipation that the central bank may eventually begin selling bonds. Too sharp a rise could bring lending to a halt and throw the economy back into a recession.

Despite the worrisome patterns observed by Stein, he added that "it need not follow that this risk-taking has ominous systemic implications."

Then again, what choice did he have? If he had said it clearly has ominous systemic implications, his speech would have gotten far more attention than he wanted. If the Fed sees a bubble forming, it wants to let the air out as slowly as possible. Thursday's speech by Stein was an attempt to do just that.